Abstract
The present work follows a previous report describing the antibacterial activity of silver camphorimine complexes of general formula [Ag(NO3)L]. The synthesis and demonstration of the antifungal and antibacterial activity of three novel [Ag(NO3)L] complexes (named 1, 2 and 3) is herein demonstrated. This work also shows for the first time that the previously studied complexes (named 4 to 8) also exert antifungal activity. The antibacterial activity of complexes was evaluated against Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia contaminans and Escherichia coli strains, while antifungal activity was tested against the Candida species C. albicans, C. glabrata, C. parapsilosis and C. tropicalis. The antimicrobial activity of the complexes ranged from very high (complex 4) to moderate (complex 6) or low (complex 8), depending on the structural and electronic characteristics of the camphorimine ligands. Notably, the highest antibacterial and anti-Candida activities do not coincide in the same complex and in some cases they were even opposite, as is the case of complex 4 which exhibits a high anti-bacterial and low antifungal activity. These distinct results suggest that the complexes may have different mechanisms against prokaryotic and eukaryotic cells. The antifungal activity of the Ag(I) camphorimine complexes (in particular of complex 1) was found to be very high (MIC = 2 μg/mL) against C. parapsilosis, being also registered a prominent activity against C. tropicalis and C. glabrata. None of the tested compounds inhibited C. albicans growth, being this attributed to the ability of these yeast cells to mediate the formation of less toxic Ag nanoparticles, as confirmed by Scanning Electron Microscopy images. The high antibacterial and anti-Candida activities of the here studied camphorimine complexes, especially of complexes 1 and 7, suggests a potential therapeutic application for these compounds.
Highlights
An increasing number of pathogenic bacterial and fungal strains resistant to existing antibiotics and antifungals have been isolated either from hospitals or the community
In this work we have investigated the activity of the three newly synthesized camphorimine Ag(I) complexes 1–3 against the ESKAPE members S. aureus, P. aeruginosa, and E. coli, the B. contaminans IST408 isolate from a Portuguese cystic fibrosis patient [29, 30]
Consistent with the increased resilience of C. glabrata species to antimicrobials [38], the MIC values of all Ag(I) complexes obtained for this species are higher than those obtained for C. tropicalis or C. parapsilosis (Table 3)
Summary
An increasing number of pathogenic bacterial and fungal strains resistant to existing antibiotics and antifungals have been isolated either from hospitals or the community This situation is a threat for public health since existing pharmaceuticals become non-effective and in some cases useless to treat infections. Infections caused by the above-mentioned bacterial and fungal species are emerging and are responsible for high rates of mortality and morbidity, posing serious challenges to the sustainability of healthcare systems [4] In this scenario, the identification of new compounds with efficacy against emerging pathogens for the development of more effective therapies [5] is a challenge to the academic community and an opportunity to the development of alternative products by the pharmaceutical industry. Albeit the toxicity has to be considered, the emergent potential of silver based species (compounds [11,12,13] or nanoparticles [14,15,16,17,18]) as antimicrobials challenge the design, synthesis, trial and assessment of the properties of new silver compounds against bacteria and fungi to explore their therapeutic and antiseptic properties (e.g. in water purification properties or on coating of implants surface [19,20,21,22])
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